Stand-alone lesson on plyr
Typos
New example for modeling across sites
Added an intro, expectations and some more examples
Addressing @jdbilshack comments in issue/91
Hard wraps and wrapped long code lines
Changes suggested by @jdblischak
Merge branch 'wking2/stand-alone-plyr-lesson'

This brings in the content from
https://github.com/swcarpentry/bc/pull/91

Thanks to @wking and @jduckles for their work on this.